Review of OCS gas-phase reactions in dark cloud chemical models

TL;DR

This paper reviews and updates the chemical reaction rates for OCS formation in dark clouds, showing that previous models underestimated the reaction rates and suggesting grain surface formation of OCS.

Contribution

It provides new ab-initio calculations and reviews of key reactions, significantly revising the predicted gas-phase OCS abundances in dark cloud models.

Findings

Revised reaction rates decrease predicted OCS abundance.

New reactions alter chemical model outcomes.

Solid OCS observed on ices supports grain surface formation.

Abstract

The association reaction S + CO {\to} OCS + hnu has been identified as being particularly important for the prediction of gas-phase OCS abundances by chemical models of dark clouds. We performed detailed ab-initio calculations for this process in addition to undertaking an extensive review of the neutral-neutral reactions involving this species which might be important in such environments. The rate constant for this association reaction was estimated to be several orders of magnitude smaller than the one present in current astrochemical databases. The new rate for this reaction and the introduction of other processes, notably OH + CS {\to} OCS + H and C + OCS {\to} CO + CS, dramatically changes the OCS gas-phase abundance predicted by chemical models for dark clouds. The disagreement with observations in TMC-1 (CP) and L134N (N), suggests that OCS may be formed on grain surfaces as is the case for methanol. The observation of solid OCS on interstellar ices supports this hypothesis.